Limonene is a monoterpene that is present in orange peel oil and has been reported to have activity against mammary (Elegbede, et al., 1984, Carcinogenesis 5: 661-664; Elegbede, et al., 1986, J. Natl. Cancer Inst. 76: 323-325; Haag, et al., 1991, Proc. Am. Assoc. Cancer Res. 32: 402); Elson, et al., 1988, Carcinogenesis 9: 331-332; Maltzman, et. al., 1989, Carcinogenesis 10: 781-783), lung, and stomach (Wattenberg, et al., 1991, Carcinogenesis 12: 115-117) cancers. (These references and all others cited herein are hereby incorporated by reference as if fully set forth herein.) Limonene has also been shown to inhibit certain skin tumors. Van Duuren et al., 1976, J. Natl. Cancer Inst., 56: 1237-1242; F. Homburger et al., 1971, Oncology, 25: 1-20.
Although studies have shown that limonene is not toxic to humans at the required usage levels, treatment with limonene is not without some side-effects, particularly when a large dose of limonene is required in a short period.